PLoS ONE

PLoS ONE Large-diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications --Manuscript Draft-Manuscript Number:

PONE-D-12-15225

Article Type:

Research Article

Full Title:

Large-diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications

Short Title:

LARGE DIAMETER BURROWS FROM ISCHIGUALASTO BASIN

Corresponding Author:

Carina E. Colombi, Ph.D. Consejo Nacional de Investigaciones Científicas y Técnicas San Juan, San Juan ARGENTINA

Keywords:

paleovertebrates; large diameter burrows; Triassic; paleoclimate; paleoecology; Pangea; seasonal climate; therapsid; ichnotaxabases; cynodont; Ischigualasto

Abstract:

Large-diameter ichnofossils comprising three morphotypes have been identified in the Upper Triassic Ischigualasto and Los Colorados formations of northwestern Argentina. The observed structures are the first of their kind to be described in detail from the Upper Triassic of South America and are the first Mesozoic forms reported from Argentina. Additionally, these burrows add to the global record of the early appearance of fossorial behavior during Late Paleozoic-Early Mesozoic time. Morphotypes 1 and 2 are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms. However, differences in diameter, overall morphology, and stratigraphic occurrence allow their independent classification. Morphotype 3 forms a complex network of straight branches that intersect at oblique angles. Their calcareous composition and surface morphology indicate these structures have a composite biogenic origin likely developed due to combined plant/animal interactions. The association of Morphotypes 1 and 2 with fluvial overbank lithologies deposited under extremely seasonal arid climate confirms interpretations that the early appearance of burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry Pangean paleoclimates. Comparisons of burrow morphology and biomechanical attributes of the abundant paleovertebrate fauna preserved in both formations permit interpretations regarding the possible burrow architects for Morphotypes 1 and 2. In the case of the Morphotype 1, the best candidate for the burrow constructor is the small carnivorous cynodont Ecteninion. Assigning an architect for Morphotype 2 is more problematic due to mismatches between the observed burrow morphology and the size of the known Los Colorados vertebrates.

Order of Authors:

Carina E. Colombi, Ph.D. Eliana Fernández Brian Currie Oscar A. Alcober Ricardo N. Martínez Gustavo A. Correa

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*Manuscript Click here to download Manuscript: Colombi et al.doc

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Large-diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications Carina E. COLOMBI1,2*, Eliana FERNÁNDEZ2, Brian S. CURRIE3, Oscar A. ALCOBER2, Ricardo MARTÍNEZ2, Gustavo CORREA1,2

1

Consejo Nacional de Investigaciones Científicas y Técnicas

2

Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan,

España 400 (Norte) San Juan, Argentina. 3

Department of Geology, 114 Shideler Hall, Miami University, Oxford, OH,

United States 45056, 513-529-7578.

*Corresponding author: [email protected]

RH: COLOMBI ET AL. – LARGE DIAMETER BURROWS FROM ISCHIGUALASTO BASIN

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Abstract Large-diameter ichnofossils comprising three morphotypes have been identified in the Upper Triassic Ischigualasto and Los Colorados formations of northwestern Argentina. The observed structures are the first of their kind to be described in detail from the Upper Triassic of South America and are the first Mesozoic forms reported from Argentina. Additionally, these burrows add to the global record of the early appearance of fossorial behavior during Late Paleozoic-Early Mesozoic time. Morphotypes 1 and 2 are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms. However, differences in diameter, overall morphology, and stratigraphic occurrence allow their independent classification. Morphotype 3 forms a complex network of straight branches that intersect at oblique angles. Their calcareous composition and surface morphology indicate these structures have a composite biogenic origin likely developed due to combined plant/animal interactions. The association of Morphotypes 1 and 2 with fluvial overbank lithologies deposited under extremely seasonal arid climate confirms interpretations that the early appearance of burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry Pangean paleoclimates. Comparisons of burrow morphology and biomechanical attributes of the abundant paleovertebrate fauna preserved in both formations permit interpretations regarding the possible burrow architects for Morphotypes 1 and 2. In the case of the Morphotype 1, the best candidate for the burrow constructor is the small carnivorous cynodont Ecteninion. Assigning an architect for Morphotype 2 is more problematic due to mismatches between the observed burrow morphology and the size of the known Los Colorados vertebrates.

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Introduction Over the last two decades, tetrapod-burrow casts with diameters greater than 10 cm have been identified across broad paleolatitudinal gradients of the supercontinent Pangea. These structures have been recorded in South Africa, Antarctica, North-America, Europe, South America and are an indicator of a relatively common tetrapod behavior during Permo-Triassic time [e.g., 1-12]. The South African burrows are particularly important because of their internal preservation of small-sized therapsid fossils, interpreted as the remains of the burrow architect [1, 2]. Despite the lack of skeletal material preserved in other Permo-Triassic burrows, these findings have been used to interpret that the burrowers of contemporaneous large-diameter burrows were also small therapsids [e.g., 4, 6, 12]. In spite of several reports of burrows from South America [e.g., 13, 7, 11], the first to be described in detail are derived from Middle Triassic strata in Brazil [12]. In this report, we build on previous reports of large-diameter burrows from the Upper Triassic Ischigualasto-Villa Union Basin of Argentina [7]. Three different types of large diameter cylindrical structures are described in detail below. Two morphotypes are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previoulsy described forms [4-6]. A third morphotype is interpreted as forming as a result of interaction between burrowing invertebrate (or vertebrate) and coeval root systems. Many researchers interpret the global Early Mesozoic appearance of the tetrapod burrows as an evolved behavioral adaptation by terrestrial vertebrates as protection against extreme climatic conditions created during the tectonic assembly and paleolatitudinal setting of the supercontinent Pangea [5, 6, 14, 9]. Low to mid-latitude Pangean climates are interpreted being highly seasonal in nature and characterized by long dry periods and a short wet season [e.g., 1519]. The burrows of the Ischigualasto-Villa Union Basin are exclusively associated with depositional facies that have been interpreted as being deposited under similar climatic conditions [20, 7]. The ichnofossils appear together with abundant vertebrates fossils in floodplain facies of high-sinuosity rivers and are associated with mature calcisols, confirming the link for earlier burrowers with extreme climatic conditions.

Geological Setting The Ischigualasto-Villa Union Basin is one of a series of the early Mesozoic continental-rift basins that formed along the southwestern margin of Pangea [21]. The fossil burrows of this report were identified in the Upper Triassic Ischigualasto and Los Colorados formations, in San Juan and La Rioja provinces, northwestern Argentina (Figure 1). In the study area the Ischigualasto Formation is comprised of ~350-700 m of fluvial channel sandstones and conglomerates, and intercalated levee, crevasse splay, and floodplain sandstones and mudstones (Figure 2) deposited by low and high sinuosity fluvial system. The formation also contains numerous layers of altered volcanic ash [22, 23]. Two of these layers, one located near the bottom and the other near the top of the Ischigualasto Formation, have been dated by

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radiometric techniques and indicate a Carnian-Norian depositional age of between ~231 and 225 Ma based on the time scale of Walker and Geissman [24] [25, 26]. The Ischigualasto Formation contains four stratigraphic members differentiated on the basis of lithological content, sedimentological architecture, and paleosol morphology [23]. In ascending order, these include: La Peña, Cancha de Bochas, Valle de la Luna and Quebrada de la Sal members (Figure 2). The taphonomic attributes of the paleoflora and paleovertebrates collected from Ischigualasto Formation differs among stratigraphic members indicating that the sedimentological/paleopedological criteria used to define the members likely developed due to changes in paleoclimatic and tectosedimentary conditions during the time of deposition [27, 28]. The large-diameter burrows of the Ischigualasto Formation described in this report come exclusively from the Cancha de Bochas Member, which is characterized by high-sinuosity channels facies interlayered with well-developed calcic paleosols that host most of the Ischigualasto paleovertebrates. This interval has been interpreted as being deposited during a period of relatively low sedimentation rates under an extremely seasonal arid climate [20, 27, 23, 26]. The Los Colorados Formation conformably overlies the Ischigualasto Formation and ranges in thickness from approximately 500 m to 700 m [29, 30]; Arcucci et al., 2004]. The unit is comprised of fluvial-channel sandstones and overbank sandstones and mudstones (Figure 3) [29]. The depositional age of the Los Colorados Formation has not been radiometric defined. However its stratigraphic continuity with the underlying Ischigualasto Formation allows assigning a Norian age. Additionally, a recent magnetostratigraphic study has reported a Norian age for the entire Los Colorados Formation [31]. The large-diameter burrows observed in Los Colorados Formation are concentrated in the upper 150 m of the unit. This interval is characterized by high-sinuosity channel deposits and associated overbank lithologies that contain mature calcic paleosols. The interval also contains most of the vertebrate fossils preserved in the Los Colorados Formation [30]. Similar to the burrowed intervals of the Ischigualasto Formation, the sedimentological, paleopedological and taphonomical characteristics of the upper Los Colorados Formation indicate low sedimentation rates and a highly seasonal xeric climatic during the time of deposition.

Paleontological Setting The Upper Triassic of Ischigualasto basin is well known by its wealth paleofaunal record, cover different habitats and sizes, including small (200 kg) tetrapods. The Carnian–Norian Ischigualasto Formation presents one of the most diversified Upper Triassic faunal records, worldwide. The formation contains several of the best known earliest dinosaurs, as well as archosauromorphs, crurotarsal archosaurs, therapsids and amphibians. Nevertheless, the diversity and abundance of fossils is variable, both laterally within individual stratigraphic intervals, and vertically through the formation. Martínez et al. [26] divided the Formation into three abundance biozones limited by local extinctions. The burrow casts form the Ischigualasto Formation are concentrated in the Scaphonyx-Exaeretodon-Herrerasaurus biozone, which is oldest

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stratigraphically, and contains the highest diversity and abundance of fossils in the formation (Table 1). Therapsids are one of the most abundant and diverse groups of vertebrates of the Scaphonyx-Exaeretodon-Herrerasaurus biozone. Among the therapsids, the cynodonts are the most diversified, as represented by the highly abundant, medium-size, herbivorous Exaeretodon and Ischignathus [32, 33], the small-size, faunivorous Ecteninion, Probelesodon, and juvenile specimen cf. Probainognathus [34-36]. The other group of therapsids are the large-size, herbivorous dicynodonts Ischigualastia [37] and Jachalleria [38]. The other group of abundant paleovertebrates is the mid-size, herbivore archosauromorph Scaphonyx [39], that represents ~60% of the fossils in this interval [26]. The biozone also includes some of the best known earliest dinosaurs, such as the small-size Pisanosaurus, Eoraptor, Panphagia, Eodromaeus, and Chromogisaurus [40, 41, 42, 26, 43], and the mid-size, herrerasaurids Herrerasaurus [44, 45] and Sanjuansaurus [46]. Scaphonyx-Exaeretodon-Herrerasaurus biozone also contains a very diverse but less abundant group of vertebrates, the crurotarsan archosaurs. This group includes the medium-size sphenosuchian Trialestes [47], the ornithosuchid Venaticosuchus [48], the proterochampsid Proterochampsa [49], and Chanaresuchus ischigualastensis [50], the armored aetosaur Aetosauroides [51], the poposaurid Sillosuchus [52] and the large-size “rauisuchid” Saurosuchus [53]. The upper Los Colorados Formation preserves a highly diverse paleovertebrate fauna characterized by its unusual arrangement of abundant dinosaurs and a high diversity of crurotarsan archosaurs. The arrangement is composed of basal crurotarsan archosaurs, dinosaurs, derived therapsids, and primitive chelonians. The most substantial change between the dinosaur fauna from the Ischigualasto Formation and that of the upper Los Colorados Formation is that the younger group expanded into the upper register of body size for both carnivores and herbivores [26]. The most abundant vertebrate in Los Colorados Formation is the largesize, basal sauropodomorph dinosaur Riojasaurus [54], that represents 40% of observed specimens. Other dinosaurs present are basal sauropodomorphs including the largesize Coloradisaurus [55] and Lessemsaurus [56, 57], as well as the uncommon, large-size, theropod Zupaysaurus [58]. Crurotarsan archosaurs are highly diverse in the Los Colorados Formation and include the medium-size, armored aetosaur Neoaetosauroides [59], the small-size sphenosuchid Pseudohesperosuchus [48] and protosuchid Hemiprotosuchus [48]; the medium-size ornithosuchid Riojasuchus [48]; and the large size rauisuchid Fasolasuchus [60, 61] Despite the relative abundance and taxonomic diversity of tetrapods from the upper Los Colorados Formation, the therapsids are relatively uncommon and are represented by the tritheledontid Chaliminia [62, 63] and a probable the tritylodontid [48]. A final minor component of the fauna of Los Colorados Formation, is the small-size, chelonian Palaeochersis [64].

Results: Large Diameter Structures

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The large-diameter (>10 cm) structures studied for this investigation are contained mainly within overbank facies of the Upper Triassic succession. Nine different stratigraphic horizons were studied in detail, including six in the Canchas de Bochas Member of the Ischigualasto Formation, and three in the upper part of Los Colorados Formation. A characterization of these structures has been made primarily on ichnotaxabases which account for the architectural and surficial morphology of the burrow casts, complexity and tortuosity indices, and the fill type [6, 65]. Evaluation of architectural morphologies includes general dimension, crosssection geometry, spatial orientation, type of branching, and burrow-element interconnectedness. Surficial morphology refers to both large and diminutive structures on the surfaces of walls. Descriptions and interpretations regarding the origin of these burrows are listed below.

Morphotype 1 This morphotype from the Ischigualasto Formation was preliminary described and interpreted as tetrapod burrow cast by Colombi et al. [7]. These burrows appear in the Cancha de Bochas Member, where five individual burrowed horizons have been identified (Figure 1 and Figure 2). The burrows are preserved in two types of overbank facies of a high-sinuosity fluvial system (Figure 2). Three of the burrow horizons are in levee facies which consist of reddish-brown colored, structureless to ripple-cross laminated, fine- to mediumgrained, muddy-sandstone and sandy-mudstone. The burrows are also developed in sandy-crevasse splay facies, characterized by structureless or ripple-cross laminated greenish-gray muddy sandstone. All facies are highly bioturbated by invertebrate burrows and overprinted by pedogenic structures including hydroximorphic mottles, root halos and traces, slicken-sided peds and abundant pedogenic carbonate nodules and rhizoconcretions [20, 23]. The Cancha de Bochas Member paleosols associated with the burrowed intervals have been interpreted as calcic vertisols, calcisols and argillic calcisols [20, 23]. The individual large-diameter burrow systems consist of horizontal to subhorizontal tunnels and short vertical shafts that cover areas of up to 2 m 2, (Figure 4). Tunnels are straight to slightly undulatory, and reach maximum lengths of 1 m (Figure 5a). The diameters of the burrows average 10 cm (with maximum diameters of ~15 cm), and display a uniform, roughly elliptical crosssection geometries. In some segments of the tunnels, the floor bears a longitudinal medial groove that forms a shallow U-shape when viewed in transverse cross section (Figure 5b). The tunnels contain intermediate and terminal enlargements, interpreted as chambers, with average diameters of ~25 cm (Figure 5c). The greater diameters of the chambers are attained by a gradual increase in tunnel diameter. Vertical shafts in burrow complexes are less than 20 cm long, although their original length may have been reduced due to compaction or erosional truncation (Figure 5d). Shafts, which likely represent burrow entrances, are commonly located at tunnel intersections or at the beginning of tunnels. The shafts are connected perpendicularly to horizontal/subhorizontal tunnels (Figure 5e). The branching angle of tunnel segments is ~90°, forming a T-shape branching (Figure 4 and 5e). The tortuosity index of the burrows (T) is 1.3, indicating the simple geometry of the branching. The complexity index of the Ischigualasto burrows is difficult to define because of incomplete burrow-cast preservation.

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The fill of the studied ichnofossils consist of brown, medium-grained, carbonate cemented sandstone. The boundaries between the burrow fill and hosting facies are very well defined. Approximately half of the observed shaft molds contain a central pit produced by differential cementation of the fill material (Figure 5d). The surficial morphology along the sides and tops of the burrows consists of poorly defined longitudinal ridges, 2-3 mm wide, which likely represent scratch marks [5]. All surfaces exhibit a granular texture created by bioturbation (Figure 5f).

Morphotype 2 These large-diameter structures appear in the upper 150 m of the Los Colorados Formation (Figures 1 and 3). They exist as cylindrical structures developed in red, structureless to weakly-laminated mudstones interpreted as overbank deposits of a meandering fluvial system. Similar to the burrowed intervals in the Ischigualasto Formation, these facies are overprinted by calcic paleosols displaying calcic nodules, scarce hydroximorphic mottles, and argilliccutans. Following interpretations of similar Ischigualasto Formation paleosols, these paleosols are classified as argillic-calcisols [20]. Burrow Morphotype 2 consist of a simple elongated network displaying one or two horizontal or subhorizontal tunnel-like structures and several vertical cylindrical structures (figure 6). The complexes cover areas of 4-8 m2 (Figure 6). Their geometries are notably simpler than the burrow complexes preserved in the Ischigualasto Formation, but have a higher density distribution within individual stratigraphic intervals. In some instances individual burrow are superimposed on older forms (Figure 6). Tunnels in this morphotype are straight or slightly curved to one side (Figure 7a). They are elliptical in cross section, with horizontal diameters averaging 35 cm (with a maximum diameter of ~50 cm) and vertical diameters averaging 20 (maximum diameter 30 cm) (Figure 7b). Tunnel floors are flattened in cross-section and display a wavy profile in longitudinal transects (Figure 7c). In horizontal segments, enlargements occur at major burrow intersections or where vertical cylinders connect with two or more horizontal segments (Figure 7d). The branching angle of Morphotype 2 tunnel-like segments is ~90° (Figure 7d), and produces a "T-shaped" branching pattern. The tortuosity index of observed horizontal segments is 1. Vertical shafts of Morphotype 2 are up to one meter in length, although original vertical dimensions may have been compressed due to sediment compaction or erosional truncation (Figure 7e). Vertical structures are commonly observed in lateral pairs displaying similar dimensions and morphological characteristics (Figure 7f and 7e). They intersect both individual and multiple horizontal-burrow segments (Figure 7d). The surface morphology Morphotype 2 is for the most part smooth. However, the base of some vertical structures display irregular vertically flattened surfaces (Figure 8). The burrow fill material consists of reddish-brown, medium-grained sandstone, cemented by carbonate. The margins between the burrow cast and the hosted rocks are very well defined due to the calcite cement and coarser-grained nature of the fill relative to the hosting mudstone. Almost all vertical structures contain a central pit produced by the differential cementation of the fill material (Figure 7f).

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The overall morphology of Morphotype 2 most closely resembles a network of tunnels and vertical shafts generated by burrowing tetrapods [cf. 2, 5-7]. The enlargements observed in the horizontal structures are interpreted as terminal and medial chambers, which preserves rugosity that resemble recent burrow development in moist or water-saturated sediments. Our interpretation of Morphotype 2 being produced by burrowing tetrapods as opposed to another potential producer of vertical/horizontal structures (e.g. crawfish, lungfish, or plant roots) is based primarily on the observed morphology of the structures. For example the large and relatively uniform diameter (~30 cm) differs from that of crayfish burrows, whose maximum reported diameter is ~8 cm [e.g. 66, 4, 67]. In addition, the relatively simple architecture of Morphotype 2, characterized by few openings to the surface, enlargements associated with two or more convergent segments, and single/coupled parallel shafts connected to undulatory horizontal tunnels, differs significantly from the usually more complicated architecture of the crayfish burrows or the bottle-like morphology of the lungfish [e.g., 66, 67]. The elliptical transverse section of Morphotype 2 is also unlike crayfish and lungfish burrows that have relatively circular cross-sectional geometries [e.g., 4, 67]. Finally, the well-defined contact between the coarser/calcite cemented burrow fill and the finer host lithologies are more consistent with a later filling of an empty burrow than a gradual filling of the space left by the progressive contraction of plant roots in the process of putrefaction as it is usually observed today.

Morphotype 3 This morphotype appears in both Ischigualasto and Los Colorados Formations in crevasse-splay deposits characterized by greenish-gray or red ripple laminated sandy mudstone (Figure 9). In plain view, Morphotype 3 forms a complex network of straight branches (tortuosity index of 1) that intersect at oblique angles of about 40° (Figure 9a). Unlike the other morphotypes, the diameter and shape of Morphotype 3 is highly irregular within and between individual structures (Figure 9b and 9c). The branches are >2.5 m in length and have elliptical/flattened-elliptical cross sectional geometries (Figure 9d) with average diameters of ~7 cm. Scarce vertical cylindrical structures with diameters up to 10 cm and more than 50 cm in length (Figure 9e) occurred in close combination with these networks of horizontal branching. Compositionally, Morphotype 3 structures are made entirely of micritic calcite containing isolated grains of sand or mud incorporated from hosting lithologies. Surfaces displays numerous millimeter diameter knobs and hummocks that resulting in an irregular texture (Figure 9f). The morphotype is associated with overlapping amalgamated micritic masses that reach 40 cm in diameter (Figure 9a and 9b). Although enigmatic, we interpret Morphotype 3 as representing composite biogenic structures developed as a result of combined plant/animal interactions. Similar structures observed in Pleistocene deposits (1.5 Ma) in east Africa are interpreted as calcified plant roots modified by animal (primarily insect) burrows that followed root channels (A. K. Behrensmeyer, personal communication, 2011). As such, Morphotype3, may represent a possible early Mesozoic example of mutualistic plant-animal interaction preserved in the fossil record.

Discussion

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The large diameter burrows in the Ischigualasto-Villa Union Basin are the first of their kind to be described in detail from the Upper Triassic of South America and are the first Mesozoic forms reported from Argentina. Additionally, these burrows add to the global record of the early appearance of fossorial behavior during Later Paleozoic-Early Mesozoic time. Morphotypes 1 and 2 described above have important implications concerning the paleobiogeographical distribution of large non-marine burrowing organisms, as well as factors controlling the evolutional development of burrowing behavior during the early Mesozoic. The first workers who investigated large diameter burrows suggested that they had a latitudinal distribution [4], as is the case of the modern burrowers which are more prevalent at higher latitudes [68-70]. However, later reports documented a diverse Permian-Triassic vertebrate ichnofossil assemblage from both low- to high-paleolatitude locations, indicating no pronounced paleolatitudinal variation at during Permo-Triassic time [e.g., 1- 3, 5-7, 9-12]. In absence of a latitudinal control on distribution of Triassic burrowing vertebrates, it has been postulated that burrowing may have been employed by early Mesozoic organisms in all latitudes in order to counter the global extreme seasonal climate [e.g., 71, 72, 8, 4, 6]. Although initially developed to combat seasonal temperature fluctuations and water stress associated with seasonally dry climate regimes at low to midlatitudes, burrowing behavior may have also allowed organisms to live at high latitudes by circumventing seasonal temperature fluctuations and perhaps serving as a refuge during winter dormancy [5-7, 73, 74]. The Upper Triassic of the Ischigualasto-Villa Union Basin was deposited under a seasonal climate as evidenced by the paleopedological [20], taphonomical [26, 27] and sedimentological studies [22, 75, 23, 76]. Moreover, the studied burrows appear exclusively associated to isolated horizons of Cancha de Bochas Member in the Ischigualasto Formation and to the upper portion of the Los Colorados Formation, where in both cases the dry and highly seasonal climate is clearly evident (i.e., calcic soils, desiccation cracks, abundance of paleovertebrates, shrubs plants restricted to temporary rivers, etc.). These conditions are in agreement with other authors who have hypothesized that burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry climates. Burrowing as an adapting mechanism to seasonal droughts was first utilized for lungfish during the Devonian [77], and this strategy may have also been employed by vertebrates in response to the development of strongly seasonal moisture variations associated with global climate change during the Permian and Triassic [e.g., 15-19]. In addition, this climate resulted in alkaline early diagenetic conditions that preserved both bone hydroxiapatite and coeval trace fossils due to the early cementation of the burrows by calcite cement [26]. In terms of paleoenvironmental evidence for the origin and preservation of the observed large-diameter burrows, the stratigraphic intervals were the burrows are observed have common sedimentological similarities. From a sedimentological standpoint, both the Cancha de Bochas and Los Colorados burrows were formed in well-drained, overbank deposits of high-sinuosity fluvial systems. During the time of burrow development, the rates of lateral fluvial channel migration and floodplain aggradation were relatively low as evidenced by the diverse and apparently temporally-condensed accumulation of vertebrate

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fossils, as well as the well-developed paleosol morphologies that characterize the interval [20, 28]. However, periodic overbank deposition and overall positive accommodation development in the basin facilitated the burial and preservation of not only fossil material, but the large-diameter burrows and individual paleosol horizons, as well. Collectively, the depositional setting of both the Cancha de Bochas and Los Colorados burrowed intervals was likely in a sedimentologically-optimized habitat for the burrowing organisms. Specifically, the areas inhabited by the burrowers were far enough removed from fluvial channels to avoid seasonally elevated water tables or flooding events, but proximal to riparian environments to take advantage of surface or groundwater sources during water-stressed periods. Identifying the possible burrowing organisms Many different morphological characteristics have been utilized to identify potential architects of continental burrows including overall burrow architecture, superficial markings, dimensions, spatial relationships, and resemblance to known tetrapod burrows [4, 5, 71, 74]. The architectural and superficial morphologies described for Morphotypes 1 and 2 above indicate the burrows were likely constructed by tetrapods. Besides the architecture, the only other parameter of these burrows that can be used to evaluate the possible burrower is the size. Burrow diameter is typically comparable to the body diameter of the animal that made it, so it is often possible to identify the burrower animal by the size of the burrow entrance [78, 79]. The burrows of Morphotype 1 in Cancha de Bochas Member of Ischigualasto Formation have an average diameter of 10 cm, with a roughly elliptical cross-section. This diameter allows fossil vertebrates from the Ischigualasto Formation with a skull width or hip height greater than 10 cm to be disregarded as the potential burrower. This limit discards the majority of the known fauna from the formation, including the herrerasaurid dinosaurs, all crurotarsal archosaurs, rhynchosaurs, amphibians, dicynodonts, and traversodontid cynodonts (see Table 1). Similarly, the small dinosaurs (Pisanosaurus, Eoraptor, Panphagia, Eodromaeus, and Chromogisaurus), with an average hip height greater than 30 cm, are still bigger than the required size. In addition, their hindlimb morphology of the small dinosaurs as slender longlegged bipedals is not consistent with the observed morphology. The only remaining burrower candidates are some of the faunivorous cynodonts identified in the Ischigualasto Formation (i.e. Ecteninion, Probelesodon, and cf. Probainognathus). Ecteninion, the most abundant faunivorous cynodont from the formation, has a skull width that varies between 3 and 8.5 cm. The larger size, presumably an adult size, is perhaps greater than that expected for the burrower. Nevertheless, the size range of burrows depends on when the animals constructed them during their ontogeny, as well as on the size range of the individuals within the same species [4]. Little is known about the habits and lifestyle of Ecteninion. Recently, however, the first post-cranial skeleton of Ecteninion lunensis has been discovered [80], allowing some of the functional aspects of the skeleton to be evaluated. Preliminary analysis of some features observed in the humerus of Ecteninion indicates similarities to those present in extant digging mammals and possible extinct fossorial cynodonts. These features include an expanded proximal and distal ends of the humerus, prominent lesser tuberosity increasing the site of insertion of the subscapularis muscle, a well-developed deltopectoral

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crest proximodistally increasing the function of the deltoideus muscle and humeral elevation and protraction, a deep biccipital groove on the humerus (like some fossorial therians), and a well-developed process for the retractor muscle (teres major) on the humerus. In addition, a preliminar mechanical analysis of the shoulder joint of Ecteninion shows that it could perform complex movements of the forearm, characterized by protraction and retraction in the horizontal plane in conjunction with elevation and depression, allowed by the simultaneous rotation of the humerus. Although these characteristics do not prove that Ecteninion was a burrower, they do not preclude this interpretation. Several authors suggested similar burrowing behavior for small to medium size nonmammalian cynodonts from South Africa and Antarctica, interpreting this behavior as a possible advantage for survival [71, 72, 8]. This may help explain the higher abundance of Ecteninion relative to other non-mammalian faunivorous cynodonts of the similar size from the Ischigualasto Formation. The burrows from Morphotype 2 of the Los Colorados Formation, with an average diameter of 30 cm, are larger than those of Morphotype 1 from Ischigualasto Formation. Based upon known fossils from Los Colorados Formation, the sauropodomorph and theropod dinosaurs, therapsid dicynodonts, quelonians, and rauisuchids were all larger than this limiting diameter and thus can be ruled out as potential architects of the burrows (see Table 1). The small to medium size aetosaurs, sphenosuchids, protosuchids, ornitosuquids, and cynodonts could be candidates as the producers and/or occupiers of the observed Los Colorados burrows. This long list of possible burrowers, however, makes the determination of an individual candidate more problematic than that for the Ischigualasto Formation burrows. Because a minimal burrow diameter is thought to be key in reducing the energetic cost of excavation, the size of the burrow is closely related to the size of the producer [81]. As such, the smaller cynodonts Chaliminia and cf. Tritylodon and the protosuchid Hemiprotosuchus from the Low Colorados Formation seem to be too small (2-3 cm of skull width) to be the burrow constructors. Other aforementioned candidates, such as the ornitosuchid Riojasuchus, the aetosaur Neoaetosauroides, and the sphenosuchid Pseudhesperosuchus, are slightly larger than the average burrow diameter. However, as noted above for the Ischigualasto burrows, it may not exclude the possibility than they used the burrows during early ontogenetic stages. Alternatively, two of the possible candidates for the architects of the Los Colorados burrows mentioned above, the cynodonts and aetosaurs, have antecedents with possible burrowing habits. The strongest evidence supporting the cynodonts as the burrowers is the discovery of several individuals of Thrinaxodon and Trirachodon in ancient burrows [71, 72, 8]. Also possible fossorial adaptations have been documented in the humerus of the cynodont Irajatherium [82], which is closely related to Chaliminia. As for the remaining group of possible burrowers, armored aetosaurs have been identified as potential constructors of the nest-holes from Petrified Forest Member of the Upper Triassic Chinle Formation [6]. In addition, some authors have speculated that the relatively massive limbs of the aetosaurs, and especially the hypertrophy of muscular trochanters, suggest enhanced muscle power related to predominantly burrowing behaviors [48, 83, 84, 85, 86].

Conclusions

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Three ichnofossil morphotypes have been identified in Upper Triassic strata of the Ischigualasto-Villa Union Basin in northwestern Argentina. The first two are interpreted as tetrapods burrow casts, while the third has been interpreted as a composite form that developed as a result of mutialistic interactions between burrowing invertebrates and coeval root systems. In spite of the widespread outcrops of Upper Triassic rocks in South America, these morphotypes are the first to be studied in detail. One significant aspect of the observed structures is their close and exclusive association with floodplain facies that display evidence of seasonal and xeric conditions. These associates are in accord with interpretations of other Permo-Triassic burrows reported at different paleolatitudes of Pangea that suggest advanced burrowing behaviors were a mechanism to combat adverse climatic conditions. In addition, based on sedimentological interpretations, it is possible to conclude that the architect of the observed burrows selected an optimum environment far enough removed from fluvial channels to avoid seasonally elevated water tables or flooding events, but proximal to riparian environments to take advantage of surface or groundwater sources during water-stressed periods. Finally, we cannot identify with absolute certainty the architects of the burrows for either the Morphotype 1 or Morphotype 2. However, for Morphotype 1, the strongest candidates are the small cynodonts Probelesodon and Ecteninion. These organisms were about the right size and have fossorial antecedents as a group, making them good candidates as the burrow architect. Unfortunately, for Morphotype 2, no one known vertebrate has the appropriate size. The cynodonts are too small, and the small to medium-size archosaurs, at least in the adult stage, are somewhat larger than burrow diameters. It cannot be ruled out, however, that the observed burrows were occupied by latter group in early ontogenetic stages.

Acknowledge We thanks Instituto y Museo de Ciencias Naturales for the support in all research stages. We also thanks to field crew of the 2005 and 2006 Ischigualasto basin fieldworks. We are also indebted to Kay Behrensmeyer for her significant field discussion.

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Figure Captions Figure 1. Study area location maps. (A) Location of the Triassic Ischigualasto-Villa Unión Basin in northwestern Argentina showing location of geologic map in Inset B (extracted from Figure 1, Colombi et al., 2008). (B) Geological map of the Triassic Ischigualasto-Villa Unión Basin showing position of satellite image shown in Inset C. (C) False color satellite image of the southern part of the basin. Red dots mark the locations of large diameter burrows identified in the Ischigualasto and Los Colorados Formations. Figure 2. Stratigraphic position of Morphotype 1 in Ischigualasto Formation. (A) Generalized stratigraphic section of the Ischigualasto Formation. Arrows indicate the stratigraphic position of the burrow casts identified in Cancha de Bochas members. (B) Photograph of typical overbank lithologies in the Cancha de Bochas Member that host the observed largediameter burrows. Figure 3. Stratigraphic position of Morphotype 2 in Los Colorados Formation. (A) Generalized stratigraphic section of the Los Colorados Formation. Arrows indicate the stratigraphic position of the burrow casts identified in the upper ~150 meters of the succession. (B) Photograph of typical fluvial channel/overbank lithologies in the upper Los Colorados Formation. Figure 4. Morphotype 1. Photograph of a general view of a typical Morphotype 1 burrow complexes (modified from Figure 3.1, Colombi et al., 2008). Note the tunnels with medial and terminal chambers (a) and the vertical shaft intersecting one of the primary tunnels (b). Figure 5. Main ichnotaxabases of the Morphotype 1 burrow complexes. (A) Straight to slightly sinuous tunnels that reach 1 meter in length. (B) Cross section of a tunnel displaying the roughly elliptical geometry and the longitudinal medial groove along the base of the burrow (modified from Figure 3.3, Colombi et al., 2008). (C) Terminal chamber with an average diameter of 25 centimeters. Note the gradual increase of the diameter from the tunnel to the chamber (modified from Figure 3.4, Colombi et al., 2008). (D) Vertical shafts in the burrow complex. Note the central pit produced by differential cementation of the fill material. (E) Relationship between shaft and the tunnels. Note the perpendicular angle between tunnels and shaft (a) (modified from Figure 3.5, Colombi et al., 2008). (F) Surficial morphology along the sides and tops of the burrows displaying the characteristic granular texture produced by bioturbation. Figure 6. Morphotype 2. Photograph of a general view of Los Colorados burrow complexes. Note the numerous exhumed vertical shafts outcropping across the landscape (arrows). Figure 7. Main ichnotaxabases of the Morphotype 2 burrow complexes. (A) Straight tunnels intersecting at right angles (arrow). (B) Elliptical cross section showing a near maximum burrow diameter of ~45 centimeters. Note the flattened tunnel base. (C) Wavy tunnel base viewed in a longitudinal section. (D) Slight enlargement at the intersection between a tunnels and a vertical

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shaft. Note the 90° angle of the tunnels intersection (arrow). (E) Vertical shafts that characterize the burrow casts preserved in Los Colorados Formation. The shafts extend over a meter above lower tunnels. (F) Common manifestation of shafts in pairs with similar morphological characteristics. See the central pits of the shafts produced by differential cementation of the fill material. The person used as scale in Figure 7A and 7E has given written informed consent, as outlined in the PLoS consent form, to publication of his photographs. Figure 8. Wrinkled surface texture of one of the Morphotype 2 burrow casts. The wrinkled texture could be produced as a consequence of likely vertical flattening of moist/saturated host sediment. Figure 9. Main ichnotaxabases of the Morphotype 3. (A) Complex network of straight branches cross-cutting at oblique angles (40°). Note the amalgamated associated calcium carbonate nodules (arrows). (B) Example of elliptical/flattened cross-sectional morphotype geometry. (C) Irregular surface of the rhizoconcretions, characterized by millimeter knobs and hummocks. (D) Horizontal branches that reach up to 3 meters in length. (E) Irregular diameter, shape, and surface morphology. (F) Vertical axes of more than 50 cm in length. Table 1. Paleofauna of Ischigualasto Formation. Sizes of different specimens are separated in three categories (< 25 kg small, 25 – 200 kg medium and >200 kg large) and identified with a green line. Table 2. Paleofauna of Los Colorados Formation. Sizes of different specimens are separated in three categories (< 25 kg small, 25 – 200 kg medium and >200 kg large) and identified with a green line.

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Table 1 Click here to download Supporting Information: Table I Ischigualasto burrows red.tif

Table 2 Click here to download Supporting Information: Table 2 Los Colorados Burrows red.tif